What Is BMI?

The body mass index, or BMI, is a metric used to estimate the amount of body fat a person has.

Though BMI doesn't measure body fat directly, it correlates with other direct measures of body fat, according to the Centers for Disease Control and Prevention (CDC).

Only two measurements are involved in BMI: height and weight. To calculate your BMI using pounds and inches, divide your weight by your height squared, and then multiply by the conversion factor of 703. (The calculation is the same using kilograms and meters, except no conversion factor is needed.)

On the BMI scale, anything below 18.5 is considered "underweight," 18.5 to 24.9 is "normal," 25.0 to 29.9 is "overweight" and an index of 30 or more is "obese."

In the health care setting, physicians use BMI to gauge if their patients are at risk for certain health issues.

For example, studies have shown that people with BMIs of 30 or more have increased risk of death from a number of diseases, including heart disease, diabetes and colorectal cancer. There are also issues with being underweight on the BMI scale, such as increased risk for malnutrition, osteoporosis and anemia.

One reason is that BMI does not take into account age and sex. Women tend to have more body fat than men of the same BMI, and older people tend to have more body fat than younger people of the same BMI, according to the CDC.

Additionally, BMI has no way of measuring where body fat is located in the body. Studies have shown that belly fat — fat around the abdominal organs — is far more dangerous than the peripheral fat beneath the skin in other areas of the body.

To correct for the index's shortcomings in determining a patient's risk of disease, the National Heart, Lung, and Blood Institute recommends using two other predictors in addition to BMI: waist circumference and risk factors associated with obesity, such as high blood pressure and physical inactivity.

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Joseph Castro

Joseph Bennington-Castro is a Hawaii-based contributing writer for Live Science and Space.com. He holds a master's degree in science journalism from New York University, and a bachelor's degree in physics from the University of Hawaii. His work covers all areas of science, from the quirky mating behaviors of different animals, to the drug and alcohol habits of ancient cultures, to new advances in solar cell technology. On a more personal note, Joseph has had a near-obsession with video games for as long as he can remember, and is probably playing a game at this very moment.